ELECTROCHEMICAL PROPERTIES OF THE UNSTABLE COMPLEXES OF CHROMIUM(III) WITH SUPEROXIDE AND HYDROPEROXIDE ANIONS AND WITH DIOXYGEN

The electrochemical behavior of the metastable (H2O)5CrOO2+ and (H2O)5CrOOH2+ cations is reported. Reproducible, irreversible cathodic responses were obtained for both ions at suitable electrodes. The formal potential of the (H2O)5CrOO2+/(H2O)5CrOOH2+ couple at pH 1 was estimated as 0.97 V vs NHE on the basis of voltammetric measurements at an oxidatively activated rotating pyrolytic graphite electrode and of the kinetics of cross-reactions with suitable, stable redox couples. The two-electron reduction of (H2O)5CrOOH2+ was observed at -0.07 V vs NHE at mercury electrodes. Comparison with the potential where H2O2 is reduced at mercury electrodes (-0.7 V vs NHE) indicates that a substantial change in the electrochemical reactivity of the peroxo ligand is produced by its coordination to the (H2O)5Cr3+ center. The possible relevance of the observed behavior to transition metal activation of O2 toward electroreduction is discussed.